accessing bitfield for sk_protocol

I'm not sure bitfields work yet. sk_protocol is a bitfield. Trying to test it:

``` C
int kretprobe__inet_csk_accept(struct pt_regs *ctx)
{
struct sock *newsk = (struct sock *)ctx->ax;
u32 pid = bpf_get_current_pid_tgid();

if (newsk == NULL)
return 0;

// check this is TCP
if (newsk->sk_protocol != IPPROTO_TCP)
return 0;
[...]
```

gets:

``` Shell
bpf: Permission denied
0: (79) r6 = *(u64 *)(r1 +80)
1: (85) call 14
2: (15) if r6 == 0x0 goto pc+71
R0=inv R6=inv R10=fp
3: (61) r1 = *(u32 *)(r6 +328)
R6 invalid mem access 'inv'
```

I also tried:

``` C
u8 protocol = 0;
bpf_probe_read(&protocol, sizeof(protocol), &newsk->sk_protocol);
```

which gets:

``` Shell
:16:46: error: address of bit-field requested
bpf_probe_read(&protocol, sizeof(protocol), &newsk->sk_protocol);
^~~~~~~~~~~~~~~~~~~
1 error generated.
Traceback (most recent call last):
File "./tcpaccept", line 101, in
b = BPF(text=bpf_text)
File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 349, in __init__
raise Exception("Failed to compile BPF module %s" % src_file)
Exception: Failed to compile BPF module
```

Start with the tcpaccept BPF snippet in the issue and reproduce both the verifier rejection and the compiler error for sk_protocol. The payload names no source files or tests, so trace how BCC handles kernel bitfields and probe reads. Done means the reported sk_protocol access works without these errors, with coverage for the reproducer.

Written by the indexing model from the issue text.